Dimmers on Low-Voltage Halogen Bulbs

It depends on how the dimming is done. If the dimmer uses pulse width modulation, then it is very efficient.

For the second question, 75W halogen vs. low voltage, it depends of the wattage of the low voltage lamps. Watts are watts, regardless of whether they come out of high or low voltage systems (although you lose a little transforming the high voltage to low voltage.) What you need to look at is light output per watt. The system that gives you the most light per watt is the most efficient.

If you have a good accurate AC ammeter and or Wattmeter, plus a good light meter you can do a series of experiments. Chart the results and compare different types of dimmers. PWM dimmers are the most efficient (GA Bhankiii). I have used PWM Dimmers for several years and have had excellent results. Incidentally, when CFL's were first introduce many years ago as a replacement for incandescant bulbs, I dutifully replaced all my lamps with them (They were reeeeaaaaly pricey back then). I was disappointed in the results and checked the light output and current draw. They gave less light per Watt than the old incandescant. I finally disposed of them and went to incandescant with dimmers as a way of saving electricity. (People thought I was nuts back then to worry about such things as power was cheap and plentiful. Now I get to say "I told you so". Or I could if everyone who called me nuts hadn't all died of old age before I got the chance.

You can check on the out put of the dimmers them selves . Most... not all will cap the out put at 80% . so there is a 20% wattage reduction there. in most peoples eyes it's hard to tell the difference between a 100w and a 75 watt bulb, most if the energy is used up in heat not much more light output. 17.1 lumen's per watt.

Guest,

1. Halogen lights are more efficient than regular incandescent lights because the filament is at a higher temperature so a larger percentage of the energy is in the visible spectrum instead of the infra-red. This is typical "black-body" performance.

2. Halogen lights can give longer life because the evaporated tungsten from the filament gets scavenged from the hot quartz bulb wall by the small amounts of halogen gas (such as iodine) which create tungsten iodide that re-deposits the tungsten on the filament. If you operate the filaments at too low a temperature, the bulb wall is also cool enough that the "halogen cycle" described above does not work, so the bulb wall darkens and light output drops. However, at these lower filament temperatures the rate of tungsten evaporation is much slower, so this problem is minor and often can be ignored entirely.

3. Lower voltage input causes lower filament temperature which moves the spectrum towards the infra-red. This reduces light output but causes a larger percent of the input energy to go to "unusable" heat, so efficiency is lower with a dimmer.

4. Filaments must be longer for higher voltages, so low-voltage bulbs can have shorter filaments and thus can be made in smaller sizes (and lower wattages). They therefore can have more precise focusing or aiming of the beam, so more of the light goes to the desired destination.

5. Task-lighting is generally more efficient than area-lighting, because we need the higher light levels only on the tasks "at hand".

Put all this together: Use halogen bulbs instead of incandescent bulbs. Wherever possible, switch from dimming to lower-wattage bulbs. Choose fixture type and voltage for the light output needed. Use task (and accent) lighting more, and area lighting less. Also, beware of fire risks because halogen bulbs operate at a higher temperature.

--John M.